In electrochemical energy devices, the use of an alkaline medium is preferred over an acidic medium primarily due to the reduced corrosion of the components of the devices, the ease of their control and use, and the ability to use less expensive construction materials. Among alkaline electrochemical energy systems, alkaline fuel cells (AFCs), metal-air batteries (e.g., zinc-air battery), and alkaline electrolyzers have been found to be more favorable due to their advantages over other alternatives, such as their higher efficiency and reduced environmental hazard.
In AFCs, because of the alkaline medium, the reduction and oxidation kinetics of oxidants and fuels (e.g., air and hydrogen) are faster for cathode and anode electrodes, respectively, thereby enabling higher power densities. In addition, the ability of using non-precious metal electro-catalysts such as nickel and silver helps to reduce the costs of the AFCs. Moreover, the less corrosive nature of an alkaline medium increases the working lifespan of AFCs.
In metal-air batteries, different metals such as lithium, zinc, aluminum, magnesium, calcium, and iron can be used as the fuel. So far, zinc (Zn) has been considered as the most logical material for the anode in a metal-air battery. This is because of the slower corrosion rate of Zn in an alkaline solution, its high electropositivity, low cost, abundance, and high produced specific energy and power density when used in a cell. In an alkaline electrolyzer, electricity is used to split water at the cathode to produce hydrogen. The advantages of an alkaline electrolyzer are the feasibility of producing highly pure products (e.g., hydrogen) and working in low temperature and a less corrosive environment.
In all of the aforementioned systems, aqueous potassium hydroxide (KOH) is used as the electrolyte, wherein the produced hydroxide ions (OH−) are conducted from the cathode to the anode. However, the existence of carbon dioxide (CO2) in air causes a problem for such systems. Specifically, such CO2 is absorbed by the KOH electrolyte and subsequently reacts with the mobile OH− ions, converting them to bicarbonate/carbonate (CO32−/HCO3−) anions. Since CO32−/HCO3− are less mobile than OH− ions, there presence results in a dramatic decrease in ion conductivity through the electrolyte.
Previous approaches to address the abovementioned deficiencies have been reported in journal articles and patent publications. The most important approach to address the problem has involved the use of solid anion exchange membranes (AEMs) instead of liquid alkaline electrolytes. Merle G. et al. (Merle, G., M. Wessling, et al. (2011). “Anion exchange membranes for alkaline fuel cells: A review.” Journal of Membrane Science 377(1): 1) reviewed numerous polymeric materials that are potentially suitable for use as an AEM and described their specific properties. However, the authors indicated that there is still a need to develop new AEMs that not only have a high ionic conductivity, but also exhibit desirable chemical stability at high pH and elevated temperatures. Various polymeric structures for AEMs, and the methods for preparing them, for use in AFCs, alkaline metal-air batteries, and alkaline electrolyzers, are disclosed in the following US Publications/Patents: 2010/0062313; U.S. Pat. No. 3,821,127; 2005/0158632; 2008/0124604; 20100239921; U.S. Pat. No. 4,663,012; and 20090306233. Nevertheless, these known membranes still lack high ionic-conductivity and durability.
Among the polymeric structures so far examined in the literature, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], or polybenzimidazole (PBI), is found to be an inexpensive, amorphous homo-polymer, having outstanding physico-chemical and thermal stability properties. Xing et al. (Xing, B. and O. Savadogo (2000). “Hydrogen/oxygen polymer electrolyte membrane fuel cells (PEMFCs) based on alkaline-doped polybenzimidazole (PBI).” Electrochemistry communications 2(10): 697-702) have shown ionic conductivities with PBI between 5×10−5 S/cm and 1×10−1 S/cm for PBI using KOH with a concentration of 6 Mat 70-90° C. However, the high anion conductivity for PBI was only achieved at elevated temperatures. In U.S. Pat. No. 5,688,613 a hydroxide conductive electrolyte based on PBI was disclosed. Such PBI film, however, does not absorb water and therefore, does not hold water within the membrane, causing it to dry out quickly.
Despite the various proposed structures as discussed above, there remains a need for an AEM that addresses at least one of the deficiencies known in the art. For example, there exists a need for a cost effective AEM, possessing (i) improved anion conductivity, (ii) improved mechanical properties, and/or (iii) improved physico-chemical and thermal stability.